The present invention relates to transport systems for webs, such as webs of PET, PEN, their copolymers and other polymers. It applies more generally to all kinds of plastic webs, laminated or coextruded plastic webs, paper webs, metallic webs such as aluminum webs, woven or non-woven webs.
Systems for transporting webs are needed, e.g. at the exit of tenter ovens or coating systems, or even inside of an oven, or for bringing the webs to the winding stations. State of the art web transport system comprise Coanda systems, where a flow of air is injected at the upstream edge of the table, parallel to the surface of the table. The web is sucked and transported downstream on the table, by this flow of air. Such systems have the following disadvantages. First, they only operate properly on a relatively short distance. Indeed, after a certain distance, the web tends to take off, flutters and does not follow the table. Second, these systems only operate on the whole width of a web, and not on pieces or parts of web. Third, they require high air volumes and only allow to obtain low forces (around 1 N/m2) and relatively low pressures.
State of the art systems also comprise Coanda systems for transporting a web along the circumference of a roll. These systems operate like the Coanda systems described above, the flat surface of the table being replaced by the cylindrical surface of the roll. These systems are subject to the same disadvantages.
There exist Coanda systems where the air is blown from holes distributed on the table; such systems are for instance sold by Hein and Lehmann, of Krefeld, Germany. The speed of air in such systems is around three times the speed of the web, while the pressure lies around 2000 Pa. The ratio of the size of the holes to the size of the active surface area lies above 9%. The flow is parallel to the table. These systems are not very efficient as they are involving a high volume of air and create an important noise because the air is hitting sharp edges..
In addition, these prior art transport systems are not appropriate for transporting ultra thin webs, e.g. submicron webs. At such thicknesses, the web has only little or no mechanical strength, so that prior art solutions cannot be used; fluttering of the web leads to high mechanical stresses to the web, and can destroy the web.
Accordingly, there exists a need for web transport systems that may provide higher forces distributed over the web, transport webs over long distances, using low air volumes, emitting minimum noise. There also exists a need for a transport system that may accommodate various widths of web, different web thickness in the range from submicron webs 1 to 36 xcexcm webs. Such a transport system should also be able to transport not only complete webs, but scrap pieces; transport speed may be as high as 500 m/min or even higher; such a system should ensure a stable transport in any given direction, and should also provide self-alignment ability. The system will operate on a flat surface (table), and also for transportation along a tube with a circular cross-section or with another cross section, e. g. a polygonal cross section. More generally, it should operate to transport webs on any surface which is essentially continuous along the transport direction.
The invention provides a solution to the above mentioned problems of the prior art; it provides a transport system having all the advantages and features listed above.
According to the invention, there is provided a table element for transporting a web, comprising:
at least one inclined blowing means;
at least one protruding hump;
at least one sucking means located between said blowing means and said protruding hump.
In a preferred embodiment, said protruding hump ensures that a web is lifted away from the table element upstream of the protruding hump. Preferably, the shape of the protruding hump ensures a partial blocking of the air blown by the blowing means, upstream of the hump.
The shape of the protruding hump may also ensure acceleration of the air flowing over the hump, downstream of the hump.
In one embodiment of the invention, said blowing means comprise lines of offset holes. The sucking means may also comprise lines of offset holes.
In another embodiment, the angle between a blowing direction of said blowing means and a direction perpendicular to the table element is comprised between 10xc2x0 and 90xc2x0, and is preferably around 30xc2x0.
It is possible that the projection on said table of a blowing direction of said blowing means forms an angle with the transport direction.
In one embodiment, the speed of the air blown by the blowing means is at least 8 times the contemplated speed of the web, preferably 10 to 15 times the speed of the web.
Preferably, the ratio of the area of the blowing means to the area of the section of the table containing the blowing means is less than 1%, preferably less than 0.5%.
Advantageously, the sucking means are located upstream of the hump, in the section where the hump cause the web to be raised.
It is possible that the protruding hump is a rounded hump. In another embodiment, the rounded hump has a circular cross section with a diameter at least eight times the thickness of the air layer between the table and the web to be transported.
The invention also relates to a transport system comprising at least one of such table element. In one embodiment, the transport system may comprise at least two series of table elements, spaced apart in a direction transverse to the transport direction. The series of transport table elements are then preferably adapted to be moved one toward or away from the other.
In one embodiment, the table elements on each side of the transport system have blowing means the blowing direction of which is directed towards the side of the transport system.
One may also provide means for pumping air to the blowing means, and from the sucking means, the ratio between the flow of air the flow of air from the sucking means and the flow of air to the blowing means being comprised between 0.5 and 1, and to preferably being around 0.7.
The invention also relates to a process for transporting webs on the surface of a table having at least one rounded hump, comprising the steps of
(a) blowing air upstream of the said at least one hump, through the table;
(b) sucking air immediately upstream of the rounded hump.
Preferably, the step of blowing air comprises blowing air in a direction inclined with respect to the surface of the table.
The invention relates to a web transport system, where a web is transported by a flow of air along the surface of a table, characterised by a hump with a shape ensuring a partial blocking of air flowing onto it.
The shape of the protruding hump will ensure acceleration of the air flowing over the hump, downstream of the hump.
The invention finally relates to a web transport system, where a web is transported by a flow of air along the surface of a table element, characterised in that the air is blown by blowing means distributed over the surface of table, at a speed at least 8 times the contemplated speed of the web, preferably 10 to 15 times the speed of the web.
Advantageously, the angle between a blowing direction of said blowing means and a direction perpendicular to the table element is comprised between 10xc2x0 and 90xc2x0, and is preferably around 30xc2x0.
In one embodiment, the projection on said table of a blowing direction of said blowing means forms an angle with the transport direction.
In another embodiment, the ratio of the area of the blowing means to the area of the section of the table containing the blowing means is less than 1%, preferably less than 0.5%.